Solid-state detectors that are highly sensitive and can be designed compactly are becoming mainstream detectors to be included in X-ray CT scanners. The structure includes a plurality of X-ray detecting elements each of which is a combination of a scintillator that convert an X-ray into light and a photodiode that converts light into an electric signal, and an electric readout circuit that reads the electric signals. The solid-state detector causes a phenomenon (afterglow) that an image formed during previous projection remains in projection data items produced during subsequent projections because of both afterglow occurring when the scintillators convert X-rays into light and signals left unread by the readout circuit.
When X-rays are irradiated, if afterglow does not occur, all signals induced by X-rays can be used to produce projection data during immediately succeeding projection (current projection). However, if afterglow occurs, all the signals cannot be used during projection immediately succeeding X-ray irradiation. Part of the signals remains over a plurality of subsequent projections. Consequently, during the projections, the signals having remained since past projections are added to signals originally acquired during current projection. This results in artifacts. The artifacts contained in projection data deteriorate image quality of reconstructed images and degrade a time resolution.
The afterglow includes a plurality of different components. This is because when the scintillator glows, the time constant of emitted light varies depending on which of a plurality of physical processes is followed from the instant the scintillator loses X-ray energy to the instant it glows. Besides, the readout circuit leaves some signals unread. Consequently, generally, afterglow is defined as the sum of components exhibiting different time constants.
Various methods have been proposed as a correction method of removing afterglow composed of a plurality of components. Proposed as one of the methods in Japanese Patent Application Laid-Open No. 6-343629 is such that a measured afterglow amount is subtracted from the result of convolution integral of the response characteristic of an X-ray detector and corrected output data items in order to compensate an adverse effect of the afterglow components occurring during respective past projections.